Differential protection of a transmission line

ABSTRACT

There is provided mechanisms for differential protection of a transmission line (20) of a transmission system (25). A method comprises obtaining a restraining current and a differential current from the transmission line (S102). The method comprises determining a compensation current for the differential current (S104). The method comprises providing the differential current as compensated for by the compensation current and the restraining current to a differential protection arrangement for making a trip decision (S106). The method comprises detecting an internal fault for the transmission system (S108). The method comprises, as a result thereof, providing the differential current without being compensated for by the compensation current and the restraining current to the differential protection arrangement for making the trip decision (S110).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application of InternationalApplication No. PCT/EP2020/050160, filed on Jan. 7, 2020, which claimspriority to European Patent Application No. 19150736.7, filed on Jan. 8,2019, which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

Embodiments presented herein relate to a method, an arrangement, acomputer program, and a computer program product for differentialprotection of a transmission line of a transmission system.

BACKGROUND

In power transmission systems, long transmission lines might have quitehigh capacitive charging currents. A real-time compensation ofcapacitive currents might therefore be needed to increase thedependability of ultra high voltage (UHV) line differential protection,especially during internal faults with high fault impedances.

Existing mechanisms for line differential protection are commonly basedon subtracting the measured or calculated differential current duringnormal operation conditions and to continue to compensate for thedifferential current even during an internal fault period, which is lesssensitive for high impedance ground faults. This is acceptable for shorttransmission line application conditions as the capacitive chargingcurrent is not so high in the short transmission line conditions for lowimpedance internal faults. When the transmission line becomes longer,especially for UHV lines, the corresponding capacitive charging currentwill be quite higher during normal operation conditions.

The existing mechanism of continuous compensation of the capacitivecharging current, even during the internal fault period, will not onlyreduce the dependency of the line differential protection, but also thecompensated charging current during internal fault condition might notbe correct. One reason for this is that the actual voltage along thetransmission line during the internal fault period might not be the sameas the case during normal operating conditions.

Hence, there is still a need for improved line differential protectionmechanisms.

SUMMARY

An object of embodiments herein is to provide efficient linedifferential protection of a transmission line of a transmission systemwhich does not suffer from the issues noted above or at least wherethese issues are reduced or mitigated.

According to a first aspect there is presented a method for differentialprotection of a transmission line of a transmission system. The methodcomprises obtaining a restraining current and a differential currentfrom the transmission line. The method comprises determining acompensation current for the differential current. The method comprisesproviding the differential current as compensated for by thecompensation current and the restraining current to a differentialprotection arrangement for making a trip decision. The method comprisesdetecting an internal fault for the transmission system. The methodcomprises, as a result thereof, providing the differential currentwithout being compensated for by the compensation current and therestraining current to the differential protection arrangement formaking the trip decision.

According to a second aspect there is presented an arrangement fordifferential protection of a transmission line of a transmission system.The arrangement comprises processing circuitry. The processing circuitryis configured to cause the arrangement to obtain a restraining currentand a differential current from the transmission line. The processingcircuitry is configured to cause the arrangement to determine acompensation current for the differential current. The processingcircuitry is configured to cause the arrangement to provide thedifferential current as compensated for by the compensation current andthe restraining current to a differential protection arrangement formaking a trip decision. The processing circuitry is configured to causethe arrangement to detect an internal fault for the transmission system.The processing circuitry is configured to cause the arrangement to, as aresult thereof, provide the differential current without beingcompensated for by the compensation current and the restraining currentto the differential protection arrangement for making the trip decision.

Advantageously this provides efficient differential protection of thetransmission line that does not suffer from the issues noted above.

Advantageously, by using adaptive compensation, the differential currentwill be compensated to very low level during normal operation conditions(and external fault conditions), whilst keeping the originaldifferential current during internal fault conditions to obtain the bestdependency and security for the line differential protection.

According to a third aspect there is presented a computer program fordifferential protection of a transmission line of a transmission system,the computer program comprising computer program code which, when run onan arrangement, causes the arrangement to perform a method according tothe first aspect.

According to a fourth aspect there is presented a computer programproduct comprising a computer program according to the third aspect anda computer readable storage medium on which the computer program isstored. The computer readable storage medium could be a non-transitorycomputer readable storage medium.

Other objectives, features and advantages of the enclosed embodimentswill be apparent from the following detailed disclosure, from theattached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted accordingto their ordinary meaning in the technical field, unless explicitlydefined otherwise herein. All references to “a/an/the element,apparatus, component, means, module, step, etc.” are to be interpretedopenly as referring to at least one instance of the element, apparatus,component, means, module, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, withreference to the accompanying drawings, in which:

FIGS. 1, 2, 3 are schematic diagrams illustrating transmission systemsaccording to embodiments;

FIGS. 4, 5, 6, 7, 8, 10 , illustrate current values according toembodiments;

FIGS. 9, 11, 12 are schematic diagrams illustrating differentialprotection arrangements, or parts thereof, according to embodiments;

FIG. 13 is a flowchart of methods according to embodiments;

FIG. 14 is a schematic diagram showing functional units of anarrangement according to an embodiment;

FIG. 15 is a schematic diagram showing functional modules of anarrangement according to an embodiment; and

FIG. 16 shows one example of a computer program product comprisingcomputer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter withreference to the accompanying drawings, in which certain embodiments ofthe inventive concept are shown. This inventive concept may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept tothose skilled in the art.

Like numbers refer to like elements throughout the description. Any stepor feature illustrated by dashed lines should be regarded as optional.

FIG. 1 schematically illustrates a transmission system 25 of a powerdistribution system where the herein disclosed embodiments apply. Thetransmission system 25 comprises at least one arrangement 10 a, 10 b fordifferential protection of a transmission line 20 of the transmissionsystem 25. Two or more arrangements 10 a, 10 b may be operativelyconnected via a communications link 23. Further, two or morearrangements 10 a, 10 b may be part of a common arrangement 10 c fordifferential protection of the transmission line 20. The arrangement 10a, 10 b may be part of, or comprise, an intelligent electronic device(IED) operating as a relay. The transmission system 25 further comprisespower sources 21 a, 21 b, current and voltage transformers 22 a, 22 b,and circuit breakers 23 a, 23 b. F1 and F2 denote external and internalfaults, respective, along the transmission line 20. The transmissionline 20 might be an ultrahigh voltage (UHV) transmission line 20. Thetransmission line 20 might be part of a power distribution system.

The embodiments disclosed herein relate to mechanisms for differentialprotection of the transmission line 20 of the transmission system 25. Inorder to obtain such mechanisms there is provided an arrangement 10 a,10 b, 10 c, a method performed by the arrangement 10 a, 10 b, 10 c, acomputer program product comprising code, for example in the form of acomputer program, that when run on an arrangement 10 a, 10 b, 10 c,causes the arrangement 10 a, 10 b, 10 c to perform the method.

The herein disclosed mechanisms overcome the above mentioned issues ofcurrent mechanisms for differential line protection by providingadaptive capacitive charging current compensation for transmission linedifferential protection. The compensation for the capacitive current is,upon detection of an internal fault, disabled so that the finaldifferential current is equal to the originally determined differentialcurrent.

Aspects of a two terminal transmission line will now be disclosed.

FIG. 2 illustrates a simplification of the transmission system of FIG. 1where the transmission line is represented as a typical two terminaltransmission system.

The related vectors in FIG. 2 indicate related positive sequence vectorsalong the transmission line, respectively: Vs1 is the sending endpositive sequence voltage, Vr1 is the receiving end positive sequencevoltage, Vm1 is the middle point positive sequence voltage, andI_(charg) is the capacitive charging current during normal loadconditions. Assuming that the load angle between Vs1 and Vr1 is δ, themiddle point positive sequence voltage Vm1 will lag half of δ withvector Vs1. I_(charg) is always 90 degrees ahead of Vm1 as indicated inthe middle of FIG. 2 . In the bottom of FIG. 2 is illustrated theequivalent positive sequence network for the given transmission system.C1 is the positive sequence leakage capacitance in total line and Z1 isthe total line positive sequence impedance.

The capacitive charging currents in each phase during normal operatingconditions, could be calculated based on the positive sequence networkas given in the bottom of FIG. 2 where C1 is the positive sequencecapacitance by considering phase A as starting phase within three phasesystems:I _(charg_a)(t)=j2πƒC ₁ Vm1(t)I _(charg_b)(t)=a ² ×I _(charg_a)(t)I _(charg_c)(t)=a×I _(charg_a)(t)  (1)

Here, a=−0.5+j0.866, which is the rotation factor for a three phasetransmission power system.

In the phasor domain, the middle point voltage Vm1 can be calculatedbased on the positive sequence network as given in the bottom of FIG. 2based on a T-type lumped equivalent circuit as given in equation (2).Here, Is1 is the positive sequence current measured in the sending end.Vm1(t)=Vs1(t)−Is1(t)×0.5Z1  (2)

For a three phase transmission line, each phase capacitive chargingcurrent can be calculated based on equations (1), (2), or alternatively,by first calculating phase A capacitive current and then rotating thephase A capacitive current with +120 degrees by multiply rotation factor“a” for phase C and −120 degrees for phase B by multiplying “a²” basedon three phase system signal relations as given in equation (1).

The calculated capacitive currents (I_(charge_a), I_(charg_b),I_(charg_c)) for each phase based on equations (1), (2) are perfectlymatching with the actual differential current (I_(d_a), I_(d_b),I_(d_c)) in each phase during normal load conditions. The differentialcurrents and restraining currents (I_(res_a), I_(res_b), I_(res_c)) ofeach phase are calculated based on the instantaneous currents assynchronized signals from both the sending end currents (I_(s_a),I_(s_b), I_(s_c)) and the receiving end currents (I_(r_a), I_(r_b),I_(r_c)), which can be obtained using equations (3)-(8) below. Usingphase A as an example, general criteria for a differential protectionfunction can be given by equation (9).I _(d_a)(t)=I _(s_a)(t)+I _(r_a)(t)  (3)I _(d_b)(t)=I _(s_b)(t)+I _(r_b)(t)  (4)I _(d_c)(t)=I _(s_c)(t)+I _(r_c)(t)  (5)I _(res_a)(t)=0.5×(|I _(s_a)(t)|+|I _(r_a)(t)|)  (6)I _(res_b)(t)=0.5×(|I _(s_b)(t)|+|I _(r_b)(t)|)  (7)I _(res_c)(t)=0.5×(|I _(s_c)(t)|+|I _(r_c)(t)|)  (8)I _(d_a_rms)(t)−k1×I _(res_a_rms)(t)>Threshold1  (9)

Here k1 is the restrain coefficient, which is below 1 and takes a valuein the range 0.2 to 0.6 in general. Threshold1 is a positive value whichis normally around 20% of nominal load current. I_(d_a_rms)(t) is theroot mean square (RMS) value of phase A differential current I_(d_a)(t)and I_(res_a_rms)(t) is the root mean square (RMS) value of phase Arestraining current I_(res_a)(t).

Aspects of a multi-terminal transmission line will now be disclosed.

In case of a multi-terminal transmission line, the same principles canbe applied. The positive sequence capacitance C1 will be then replacedby the total summation of the connected line positive sequencecapacitances denoted C_(1Σ). The middle point voltage in the positivesequence network will be calculated based on the equivalent positivesequence network. The middle point will be along the transmission linebetween the two most separated terminals within the multi-terminaltransmission line system. The total equivalent capacitance can becalculated as below in equation (10) if assumed that there are “M”terminals connected in the differential zone:C _(1Σ)=Σ_(k=1) ^(M) C _(1k)  (10)

The total charging current (using phase A as example) can be calculatedas in equation (11) below.I _(charg_a)(t)=j2πƒC _(1Σ) Vm1(t)  (11)

Here, Vm1(t) is the middle point voltage in the positive sequencenetwork along the transmission line between the two most separatedterminals within the multi-terminal transmission line system. Capacitivecurrents for phases B and C could also be obtained by using the samerotation factors as indicated in equation (1).

An example of a three terminal connected network is shown in FIG. 3 .Three sources are connected as a three-terminal system. Here the longestline between two terminals is the transmission line between source S andsource R. The middle point is 250 km in this longest transmission line.Vs1, Vr1, Vt1, Is1, and It1 are the related positive sequence voltagesand currents for the calculation. Z1 is the positive sequence impedancefor the 500 km overhead line. If different types of transmission linesare used in the multi-terminal line system, corresponding modificationof the positive sequence network impedance values need to be modified.

Based on the positive sequence network, the middle point voltage Vm1 canbe calculated as expressed in equation (12) below and the finalcapacitive charging current can be calculated based on equations (10)and (11).Vm1(t)=Vs1(t)−Is1(t)×0.2Z1−(Is1(t)+It1(t))×0.3Z1  (12)

Aspects of real time capacitive current compensation for linedifferential protection applications will now be disclosed byconsidering the phase A loop as an example.

The real time capacitive current calculation and compensation could beused for line differential protection, which can improve the securityfor line differential protection as the compensation could compensatethe differential current to almost zero level during normal loadconditions. The compensation could be obtained by using equation (13)below for each phase respectively:I _(d_comp_a)(t)=I _(d_a)(t)−I _(charg_a)(t)I _(d_comp_b)(t)=I _(d_b)(t)−I _(charg_b)(t)I _(d_comp_c)(t)=I _(d_c)(t)−I _(charg_c)(t)  (13)

On the other hand, the compensation could be either switched off in caseof disturbance created by the internal faults so that the differentialcurrent will be equal to the actual differential current or controlledin case of external fault conditions

FIG. 4 shows the internal fault condition with phase A to ground fault(solid ground fault) by considering the phase A loop as an example. Herethe signal final_ID is the compensated differential current in phase A.Ida1 (Ida1=Ida(t)) is the original differential current (uncompensateddifferential current) in phase A. It is shown that the compensateddifferential current final_ID is almost zero during normal condition andthen it is switched back to original differential current Ida1 once aninternal fault is detected. Ida1rms (Ida1rms=Id_a_rms(t)) is the rootmean square value (RMS) of differential current in phase A. Iresrms(Iresrms=Ires_a_rms(t)) is the RMS value for restraining current forphase A. Final_IDrms is the RMS value of compensated differentialcurrent in phase A. The signal Diff_Trip is the final trip signal forthe internal fault condition. In this case, the trip signal is set tovalue 1 after 5 ms based on general differential protection restraincharacteristics. It is clear that the compensation has created positiveeffect for the security of line differential protection during normalcondition and it does not influence the dependency of protection duringinternal solid fault condition.

For internal fault with high impedance fault condition, the compensateddifferential current will be reduced a lot because the high impedancefault does not create obvious changes both for voltages and currents.The differential and restraining currents in each phase will thus havelimited changes. FIG. 5 shows the high impedance fault in phase A withfault impedance as 1000 ohms. A trip signal Diff_Trip is set to value 1after 10 ms. As seen in FIG. 5 , the original differential current Ida1(Ida1=Ida(t)) in phase A does not change so much during the internalfault period for the high impedance ground fault. On the other hand, thecapacitive leakage current for the given transmission line is almostclose to 1000 Ampere peak value. If the compensation for thedifferential current Id_comp (Id_comp=Id_comp_a(t)) is kept, it willreduce the dependency of the differential protection during faultperiod. The related RMS value is Id_com_rms. The dependency of thedifferential protection will be influenced. In this condition, it isadvantageous to switch back to the original differential current oncethe internal fault is detected as shown in FIG. 5 with signals final_IDand final_IDrms. In this way, better dependency is obtained.

This high impedance fault condition during internal fault in phase A isfurther illustrated with reference to FIG. 6 . The vector diagram inFIG. 6 -(a) indicates the fault period differential current (IA_diff)and related capacitive charging current (IA_diff_C) and fault resistancecurrent (IA_diff_R). FIG. 6 -(b) indicates the instantaneous signals andRMS value of those signals. From the middle window of FIG. 6 -(b) it canbe seen that the compensated differential current RMS value(Id_comp_rms) is much lower than the uncompensated differential currentRMS value idairms (idcurms=Id_a_rms(t)). In the illustrative example ofFIG. 6 , Ia_diff_R=425 A, Ia_diff_C=900 A, Ia_diff=995 A, andId_comp_rms=425 A. It can also be seen that the uncompensateddifferential current angle is 90 degrees ahead of the middle pointpositive sequence voltage vector angle and it is 61 degrees ahead of themiddle point positive sequence voltage during fault period which is inline with FIG. 6 -(a). FIG. 6 -(c) gives an equivalent circuit of phaseA to ground fault with high impedance Rf. Here Ceq is the equivalentpositive sequence capacitance in phase A. Vaf is the phase A to groundvoltage, which is close to the measured Vm1 for the high impedancefault.

For external faults, theoretically, the differential current will bezero based on sending end and receiving end current directionalities. Inpractice, especially for long transmission line conditions, the finaldifferential current as seen in FIG. 7 as Ida1 is not zero during theexternal fault period with solid ground fault due to possible errorscreated by current transformer saturation, measurement errors, etc. Froma security point of view, it can be advantageous to keep thedifferential current to zero. An external fault detection switch mighttherefore be used to switch the differential current to zero once anexternal fault is detected. Another way is to keep the differentialcurrent as original differential current as Ida1 if an external fault isdetected.

Aspects of internal fault and external fault detection will now bedisclosed.

For internal fault detection, the following criteria as given below inequations (14), (15), (16) can be used by considering phase A as anexample. Here, t is the time instant and T is one fundamental cycle timefor related power systems.ΔI _(d_a)(t)=ΔI _(da)(t)=I _(d_a)(t)−I _(d_a)(t−T)  (14)ΔI _(res) _(a) (t)=ΔI _(ra)(t)=I _(res_a)(t)−I _(res_a)(t−T)  (15)ΔI _(da_angle)(t)=ΔI _(da_angle)(t)=I _(d_a_angle)(t)−I_(d_a_angle)(t−T)  (16)

Here, equation (14) is the calculation for changes of differentialcurrent in phase A, equation (15) is the calculation for changes ofrestraining current in phase A, and equation (16) is the calculation ofangle changes in differential current in phase A.

The internal fault might create a sudden increase of differentialcurrents, or restraining currents, in the related fault phases. Inparallel, the differential current angle will be decreased from thetotal capacitive current condition to a combination of both capacitivecurrent with resistive current, as shown in FIG. 6 -(a). The decrease ofthe differential current angle will depend on the fault resistancelevel. The equations (14)-(16) are general expressions by using phase Aas an example, which can be applied for each phase calculations for thesudden changes of differential currents, restraining currents and anglesof differential currents and it is possible to use alternative methodssuch as using two cycle data instead of one cycle data to obtain abovesudden change values.

FIG. 8 and FIG. 9 show that the internal fault detection switch can besuccessfully used for an internal fault case and show that the logicworks both for solid faults and high impedance faults. With respect toFIG. 12 , which will be described below, FIG. 9 illustrates a scheme 900of the internal fault detection switch by considering phase A as anexample. In case of internal faults, the internal fault switch will beset to 1 so that the compensation will be disabled, and the actualdifferential current will be used in the differential protection. Inthis way, the sensitivity of the differential protection is increased.

For external faults, the corresponding differential currents of thefaulty phases will decrease immediately due to the current directionalchanges in the external faults. The restraining currents will increaseimmediately following the fault inception because of high fault currentsfeeding to the fault point. The changes of differential currents andrestraining currents in the faulted phases can be used to detectexternal faults. The basic concept is to detect the decrease ofdifferential current and sudden increase of restraining current thatwill give an efficient identification of external faults. For a currenttransformer (CT) saturation condition, it is still possible to detectthe external fault if the CT will not saturate within 1-2 ms after eachzero-crossing point. This condition for most of line protection schemewill be fulfilled because line CTs do not have big ratio differences.FIG. 10 and FIG. 11 show an example for the external fault in the busbarof receiving end substation as shown in FIG. 2 . With respect to FIG. 12, which will be described below, FIG. 11 illustrates a scheme 1100 ofthe external fault detection switch by considering phase A as anexample. When an external fault is detected, the external fault switchwill be set to 1 so that the differential current will be set to zero.

The overall differential protection scheme 1200 with adaptive capacitivecurrent compensation in the arrangement 10 a is shown in FIG. 12 byusing phase A as an example. In FIG. 12 , Set 1 is the threshold valuefor instantaneous differential protection criterion, as performed in adifferential protection arrangement 1210, and Set2 is the thresholdvalue for the RMS based differential protection criterion. K1 is a ratiovalue which is normally set to 0.5. The arrangement 10 a comprises anexternal fault detection switch (as illustrated in FIG. 11 in moredetail) and an internal fault detection switch (as illustrated in FIG. 9in more detail). When Ctrl=1 for the external fault detection switch itis set to position A, and else it is set to position B. When Ctrl=1 forthe internal fault detection switch it is set to position A, and else itis set to position B. Hence, the two signals Ctrl can be used to controlthe position of the external fault detection switch and the position ofthe internal fault detection switch.

I_(d_comp_a)(t) is determined from I_(d_a)(t) and I_(charg_a)(t)according to equation (13).

Further, Final_Ida(t) is the real time differential current for phase Aand the RMS value of Final_Ida(t) is denoted Final_Ida_rms(t) and iscontinuously calculated based on fundamental power frequency cycle timeT as given below in equation (17).

$\begin{matrix}{{{Final\_ Ida}{\_ rms}(t)} = \sqrt{\frac{1}{T}{\int_{t - T}^{t}{\left( {{Final\_ Ida}(t)} \right)^{2}dt}}}} & (17)\end{matrix}$

FIG. 13 is a flowchart illustrating embodiments of methods fordifferential protection of a transmission line 20 of a transmissionsystem 25. The methods are performed by the arrangement 10 a, 10 b, 10c. The methods are advantageously provided as computer programs 1620.

S102: A restraining current I_(res) and a differential current I_(d) areobtained from the transmission line 20.

The restraining current I_(res) and the differential current I_(d) ofeach phase A, B, C be calculated as in equations (3)-(8). During normalconditions, i.e., when no fault occurs, the differential currentrepresents a transmission line leakage current for the transmissionsystem 25.

S104: A compensation current is determined for the differential current.According to an embodiment the compensation current is a capacitivecompensation current.

S106: The differential current as compensated for by the compensationcurrent and the restraining current are provided to the differentialprotection arrangement 1210 for making a trip decision. With referenceto FIG. 12 , the internal fault detection switch is thus set to positionA, and the external fault detection switch is set to position B.

S108: An internal fault F2 is detected for the transmission system 25.

S110: As a result of the internal fault having been detected (as in stepS108) the differential current without being compensated for by thecompensation current and the restraining current are provided to thedifferential protection arrangement 1210 for making the trip decision.With reference to FIG. 12 , the internal fault detection switch is thusset to position B, and the external fault detection switch is set toposition B.

In this respect, in a three phase AC transmission system, thedifferential currents and restraining currents are phase segregated,which means that there are three differential currents and threerestraining currents. Each phase will have one differential current andone restraining current and hence there is one differential protectionscheme per phase. These three differential protection schemes (one ineach phase) are run in parallel to define the overall differentialprotection scheme. As an example, if there is a fault in phase A, thedifferential protection function for phase A will detect the fault, andtrip phase A. As a further example, if there is a fault involved withphase A and phase B, both the differential protection schemes for phaseA and phase B will detect the faults and send trips to the circuitbreakers for phase A and phase B to isolate the fault, and so on.

Embodiments relating to further details of differential protection ofthe transmission line 200 of the transmission system 25 as performed bythe arrangement 10 a, 10 b, 10 c will now be disclosed.

The compensation current might be determined such that during normaloperation the compensated differential current is zero. With referenceto FIG. 12 , the external fault detection switch is thus set to positionA.

As disclosed above, e.g., with reference to FIG. 2 , the transmissionsystem 25 is representable as a positive sequence network. Therestraining current I_(res) and the differential current I_(d) mightthen be obtained by being calculated from parameters of the positivesequence network.

In some aspects an external fault F1 is detected and hence steps S112and S114 are performed:

S112: An external fault F1 is detected for the transmission system 25.

S114: As a result of the external fault having been detected thedifferential protection arrangement 1210 is disabled from making thetrip decision. In some embodiments, the differential protectionarrangement 1210 is disabled from making any trip decision by providingthe differential current as set to zero and the restraining currentbeing provided to the differential protection arrangement 1210 formaking the trip decision. With reference to FIG. 12 , the external faultdetection switch is thus set to position A.

It might here be assumed that if a fault occurs, the fault is either aninternal fault F2 or an external fault F1. This does not exclude thattwo or more faults might occur time-wise one after the other, where thetime-wise first occurring fault is either an internal fault F2 or anexternal fault F1, and where the time-wise second occurring fault iseither an internal fault F2 or an external fault F1, and so on. Whichaction, or step, to be perform thus depends on what type of fault isdetected.

Thus, an alternative way of formulating the invention is to, upon havingobtained the restraining current I_(res) and a differential currentI_(d) determining whether to apply a compensation current or not (beforebeing provided to the differential protection arrangement 1210 formaking the trip decision) depending on whether no fault has beendetected, whether an internal fault has been detected, or whether anexternal fault has been detected. The compensation current is notapplied when the internal fault has been detected. The differentialprotection arrangement 1210 is disabled from making any trip decisionwhen the external fault has been detected. The differential currentmight be set to zero when the external fault has been detected. Thecompensation current is applied when no fault has been detected. Thiscan be achieved by setting the positions of the external fault detectionswitch and the internal fault detection switch as disclosed above.

Hence, according to an alternative formulation of the invention, therestraining current I_(res) and the differential current I_(d) areobtained. It is then checked whether no fault has been detected, aninternal fault has been detected, or an external fault has beendetected. When no fault has been detected, a compensation current isdetermined and the differential current as compensated for by thecompensation current and the restraining current are provided to thedifferential protection arrangement 1210 for making a trip decision.When an internal fault has been detected the differential currentwithout being compensated for by any compensation current and therestraining current are provided to the differential protectionarrangement 1210 for making the trip decision. When an external faulthas been detected the differential current as set to zero and therestraining current are provided to the differential protectionarrangement 1210 for making the trip decision. In this respect, duringexternal faults, the restraining current, which is summation of theaverage absolute values of all terminal currents, will be higher. Sincethe differential current is zero, the differential protection based onequation (9), that is, the conditionI_(d_a_rms)(t)−k1×I_(res_a_rms)(t)>Threshold1 will not be fulfilled,hence ensuring security of the differential protection. Again, this canbe achieved by setting the positions of the external fault detectionswitch and the internal fault detection switch as disclosed above.

The restraining current for an M-terminal transmission system 25 at timet is denoted I_(res,x)(t) and is defined as:

$\begin{matrix}{{{I_{{res},x}(t)} = \frac{\left( {{{I_{s,1,x}(t)}} +} \middle| {I_{s,2,x}(t)} \middle| {{+ \ldots} + {{I_{s,M,x}(t)}}} \right)}{M}},} & (18)\end{matrix}$where I_(s,m,x)(t) represents instantaneous current at time t fromterminal m along the transmission line 20 for phase x, where x∈{A, B, C}for a 3-phase transmission system 25 with phases A, B, C.

FIG. 14 schematically illustrates, in terms of a number of functionalunits, the components of an arrangement 10 a, 10 b, 10 c fordifferential protection of the transmission line 20 of the transmissionsystem 25 according to an embodiment. Processing circuitry 1410 isprovided using any combination of one or more of a suitable centralprocessing unit (CPU), multiprocessor, microcontroller, digital signalprocessor (DSP), etc., capable of executing software instructions storedin a computer program product 1610 (as in FIG. 16 ), e.g. in the form ofa storage medium 1430. The processing circuitry 1410 may further beprovided as at least one application specific integrated circuit (ASIC),or field programmable gate array (FPGA).

Particularly, the processing circuitry 1410 is configured to cause thearrangement 10 a, 10 b, 10 c to perform a set of operations, or steps,as disclosed above. For example, the storage medium 1430 may store theset of operations, and the processing circuitry 1410 may be configuredto retrieve the set of operations from the storage medium 1430 to causethe arrangement 10 a, 10 b, 10 c to perform the set of operations. Theset of operations may be provided as a set of executable instructions.

Thus the processing circuitry 1410 is thereby arranged to executemethods as herein disclosed. The storage medium 1430 may also comprisepersistent storage, which, for example, can be any single one orcombination of magnetic memory, optical memory, solid state memory oreven remotely mounted memory. The arrangement 10 a, 10 b, 10 c mayfurther comprise a communications interface 1420 at least configured forobtaining current values from the transmission system 25, to providecurrent values to the differential protection arrangement 1210, and forcommunications with another arrangement 10 a, 10 b, 10 c. As such thecommunications interface 1420 may comprise one or more transmitters andreceivers, comprising analogue and digital components. The processingcircuitry 1410 controls the general operation of the arrangement 10 a,10 b, 10 c e.g. by sending data and control signals to thecommunications interface 1420 and the storage medium 1430, by receivingdata and reports from the communications interface 1420, and byretrieving data and instructions from the storage medium 1430. Othercomponents, as well as the related functionality, of the arrangement 10a, 10 b, 10 c are omitted in order not to obscure the concepts presentedherein.

FIG. 15 schematically illustrates, in terms of a number of functionalmodules, the components of an arrangement 10 a, 10 b, 10 c fordifferential protection of the transmission line 20 of the transmissionsystem 25 according to an embodiment. The arrangement 10 a, 10 b, 10 cof FIG. 15 comprises a number of functional modules; an obtain module1510 a configured to perform step S102, a determine module 1510 bconfigured to perform step S104, a provide module 1510 c configured toperform step S106, a detect module 1510 d configured to perform stepS108, and a provide module 1510 e configured to perform step S110.

The arrangement 10 a, 10 b, 10 c of FIG. 15 may further comprise anumber of optional functional modules, such as any of a detect module151 of configured to perform step S112, and a disable module 1510 gconfigured to perform step S114. In general terms, each functionalmodule 1510 a-1510 g may in one embodiment be implemented only inhardware and in another embodiment with the help of software, i.e., thelatter embodiment having computer program instructions stored on thestorage medium 1430 which when run on the processing circuitry makes thearrangement 10 a, 10 b, 10 c perform the corresponding steps mentionedabove in conjunction with FIG. 15 . It should also be mentioned thateven though the modules correspond to parts of a computer program, theydo not need to be separate modules therein, but the way in which theyare implemented in software is dependent on the programming languageused. Preferably, one or more or all functional modules 1510 a-1510 gmay be implemented by the processing circuitry 1410, possibly incooperation with the communications interface 1420 and/or the storagemedium 1430. The processing circuitry 1410 may thus be configured tofrom the storage medium 1430 fetch instructions as provided by afunctional module 1510 a-1510 g and to execute these instructions,thereby performing any steps as disclosed herein.

FIG. 16 shows one example of a computer program product 1610 comprisingcomputer readable storage medium 1630. On this computer readable storagemedium 1630, a computer program 1620 can be stored, which computerprogram 1620 can cause the processing circuitry 1410 and theretooperatively coupled entities and devices, such as the communicationsinterface 1420 and the storage medium 1430, to execute methods accordingto embodiments described herein. The computer program 1620 and/orcomputer program product 1610 may thus provide means for performing anysteps as herein disclosed.

In the example of FIG. 16 , the computer program product 1610 isillustrated as an optical disc, such as a CD (compact disc) or a DVD(digital versatile disc) or a Blu-Ray disc. The computer program product1610 could also be embodied as a memory, such as a random access memory(RAM), a read-only memory (ROM), an erasable programmable read-onlymemory (EPROM), or an electrically erasable programmable read-onlymemory (EEPROM) and more particularly as a non-volatile storage mediumof a device in an external memory such as a USB (Universal Serial Bus)memory or a Flash memory, such as a compact Flash memory. Thus, whilethe computer program 1620 is here schematically shown as a track on thedepicted optical disk, the computer program 1620 can be stored in anyway which is suitable for the computer program product 1610.

The inventive concept has mainly been described above with reference toa few embodiments. However, as is readily appreciated by a personskilled in the art, other embodiments than the ones disclosed above areequally possible within the scope of the inventive concept, as definedby the appended patent claims.

The invention claimed is:
 1. A method for differential protection of atransmission line of a transmission system, the method comprising:obtaining a restraining current and a differential current from thetransmission line; determining a compensation current for thedifferential current; providing the differential current as compensatedfor by the compensation current and the restraining current to adifferential protection arrangement for making a trip decision;detecting an internal fault for the transmission system; and in responseto detecting the internal fault, providing the differential currentwithout being compensated for by the compensation current and therestraining current to the differential protection arrangement formaking the trip decision.
 2. The method according to claim 1, furthercomprising: detecting an external fault for the transmission system; andin response to detecting the external fault, disabling the differentialprotection arrangement from making any trip decision.
 3. The methodaccording to claim 2, wherein the differential protection arrangement isdisabled from making any trip decision by providing the differentialcurrent as set to zero and the restraining current to the differentialprotection arrangement for making the trip decision.
 4. The methodaccording to claim 1, wherein the transmission system is representableas a positive sequence network and wherein the restraining current andthe differential current are obtained by being calculated fromparameters of the positive sequence network.
 5. The method according toclaim 1, wherein the compensation current is a capacitive compensationcurrent.
 6. The method according to claim 1, wherein the restrainingcurrent for an M-terminal transmission system at time t is denotedI_(res,x) (t) and is defined as:I_(res,x)(t)=((|I_(s,1,x)(t)|+|I_(s,2,x)(t)|+ . . . +|I_(s,M,x)(t)|))/M,where I_(s,m,x) (t), with m ranging from 1 to M, representsinstantaneous sent current at time t from terminal m along thetransmission line for phase x, where x∈{A,B,C} for a 3-phasetransmission system with phases A,B,C.
 7. The method according to claim1, wherein the compensation current is determined such that duringnormal operation the compensated differential current is zero.
 8. Themethod according to claim 1, wherein the transmission line is an ultrahigh voltage transmission line.
 9. The method according to claim 1,wherein the transmission line is part of a power distribution system.10. An arrangement for differential protection of a transmission line ofa transmission system, the arrangement comprising processing circuitry,the processing circuitry being configured to cause the arrangement to:obtain a restraining current and a differential current from thetransmission line; determine a compensation current for the differentialcurrent; provide the differential current as compensated for by thecompensation current and the restraining current to a differentialprotection arrangement for making a trip decision; detect an internalfault for the transmission system; and in response to detecting theinternal fault, provide the differential current without beingcompensated for by the compensation current and the restraining currentto the differential protection arrangement for making the trip decision.11. The arrangement according to claim 10, wherein the processingcircuitry further is configured to cause the arrangement to: detect anexternal fault for the transmission system, and in response to detectingthe external fault, disable the differential protection arrangement frommaking any trip decision.
 12. The arrangement according to claim 11,wherein the processing circuitry further is configured to cause thearrangement to disable the differential protection arrangement frommaking any trip decision by providing the differential current as set tozero and the restraining current to the differential protectionarrangement for making the trip decision.
 13. The arrangement accordingto claim 10, wherein the transmission system is representable as apositive sequence network, and wherein the processing circuitry furtheris configured to cause the arrangement to obtain the restraining currentand the differential current by calculating from parameters of thepositive sequence network.
 14. The arrangement according to claim 10,wherein the compensation current is a capacitive compensation current.15. The arrangement according to claim 10, wherein the restrainingcurrent for an M-terminal transmission system at time t is denotedI_(res,x) (t) and is defined as:I_(res,x)(t)=((|I_(s,1,x)(t)|+|I_(s,2,x)(t)|+ . . . +|I_(s,M,x)(t)|))/M,where I_(s,m,x) (t), with m ranging from 1 to M, representsinstantaneous sent current at time t from terminal m along thetransmission line for phase x, where x∈{A,B,C} for a 3-phasetransmission system with phases A,B,C.
 16. The arrangement according toclaim 10, wherein the compensation current is determined such thatduring normal operation the compensated differential current is zero.17. The arrangement according to claim 10, wherein the transmission lineis an ultra high voltage transmission line.
 18. The arrangementaccording to claim 10, wherein the transmission line is part of a powerdistribution system.
 19. The arrangement according to claim 10, whereinthe processing unit is part of an Intelligent Electronic Devicecomprised in the arrangement.
 20. A computer program for differentialprotection of a transmission line of a transmission system, the computerprogram comprising computer code which, when run on processing circuitryof an arrangement, causes the arrangement to: obtain a restrainingcurrent and a differential current from the transmission line; determinea compensation current for the differential current; provide thedifferential current as compensated for by the compensation current andthe restraining current to a differential protection arrangement formaking a trip decision; detect an internal fault for the transmissionsystem; and in response to detecting the internal fault, provide thedifferential current without being compensated for by the compensationcurrent and the restraining current to the differential protectionarrangement for making the trip decision.
 21. A computer program productcomprising a non-transitory computer readable storing the computerprogram according to claim 20, and a computer readable storage medium onwhich the computer program is stored.